1. Field of the Invention
This invention relates to high power semiconductor devices and more particularly, to improved heat sinks for heat management of high power semiconductor light emitters and power devices.
2. Description of the Related Art
Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light energy and generally comprise an active layer of semiconductor material sandwiched between two oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate photons and phonons. Photons are useful because they provide radiative recombination or light, which is typically emitted omnidirectionally from the active layer and from the surfaces of the LED.
LEDs have the potential to provide replacement for long-standing illumination technologies such as incandescent and fluorescent lighting. In comparison to these mature technologies, LEDs are longer-lasting, physically more rugged, use less power, and are more efficient. Historically, however, LEDs have lacked brightness comparable to incandescent, fluorescent or vapor-discharge lights and thus these older technologies have continued to occupy the field. Only recently, have LEDs begun to make inroads into commercial lighting applications, with most of these being in smaller applications such as flashlights, intersection signal lights and automotive tail lights.
Despite generating light more efficiently, conventional LEDs dissipate approximately 50-70% of their input power as heat, which is generated due to the finite series resistance of LEDs in operation and to non-radiative recombination in LEDs with quantum efficiency less than 100%. The wall plug efficiency of LEDs determines their energy conversion efficiency. It is calculated as the ratio of the light output power emitted by the LED and the electrical power required to generate that light. For example, a relatively efficient LED has a series resistance of 1 ohm, an operating voltage of 4.0 volts at an operating current of 1 ampere and a wall plug efficiency of approximately 30%. The input power for this LED at 1 ampere is 4 watts. At 30% wall plug efficiency the LED emits 1.2 watts radiant energy as light. The energy balance of 2.8 watts is converted to heat. This level of heat dissipation can cause LEDs to operate at relatively high temperatures.
As LEDs are more frequently being used for commercial applications, it will be necessary for them to produce a higher luminous flux. These high luminous flux devices will be required to produce illumination that reaches several hundred to a few thousand lumens. One way to increase a devices luminous flux is to increase the level of the input power applied to the devices. To produce the desired luminous flux, the input power can be increased up to several watts to tens of watts, which will in turn cause the LEDs or LED arrays to operate at higher temperatures. Higher operating temperatures cause the device quantum efficiency to decrease. Further, the device's lifespan can also be decreased due to accelerated materials degradation.
One way to decrease the operating temperature of an LED is to mount it in thermal contact with a heatsink. For high power LEDs the operating temperature can be reduced to an acceptable level using large heat sinks that are typically fabricated from a bulk metal, such as aluminum. The heat sink operates as a high thermal conductivity path to conduct heat away from the LED and eventually dissipates the heat to the ambient. Typical heat sinks have features such as fins that increase the heat sink's surface to provide for greater convection heat exchange with the ambient air. These larger heat sinks can consume an unacceptable amount of space and can also be prohibitively heavy, expensive and complicated to manufacture. Further, bulky heat sinks reduce the flexibility in the design of LED systems.